A plate heat exchanger comprises a plate pack consisting of a number of assembled heat transfer plates forming between them plate interspaces. In most cases, every second plate interspace communicates with a first inlet channel and a first outlet channel, each plate interspace being adapted to define a flow area and to pass a flow of a first fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with a second inlet channel and a second outlet channel for a flow of a second fluid. Thus, the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a considerable heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of sheet bars that have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more “ports” consisting of through holes punched in the plate. The ports of the different plates define said inlet and outlet channels, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or any other form of sealing means are alternatingly arranged around some of the ports in every second plate interspace and, in the other plate interspaces, around the other ports so as to form the two separate channels for the first fluid and the second fluid, respectively.
Since the fluid pressure levels attained in the heat exchanger during operation are considerable, the plates need to have a certain rigidity so as not to be deformed by the fluid pressure. The use of plates made of sheet bars is possible only if the plates are somehow supported. As a rule, this is solved by the heat transfer plates being designed with some kind of pattern so that the plates bear against each other in a large number of points. The plates are clamped together between two rigid end plates in a “frame” and thereby form rigid units having flow channels in each plate interspace. To obtain the desired contact between the plates, two different types of plates are manufactured, which are then alternatingly arranged in such manner that the plates in the heat exchanger are alternately of a first kind and of a second kind. Alternatively, use is made of identical plates which alternately are turned or flipped about a symmetry axis.
In most cases, the ports for the respective flow areas are located in two port portions at two opposite edges of the heat transfer plate, and said flow areas are formed by a heat transfer surface located between the port portions. In the portion of the plated located closest to the ports (the distribution surface), the plates usually have a pattern which has been specially designed to distribute the fluids over the entire width of the flow area.
In some applications, the pressure drop across the heat transfer surface represents only a small part of the pressure drop, which means that the difference in pressure drop in the transverse direction will be relatively small even if relatively large differences in fluid flow would arise across the width of the flow area. Although an uneven distribution, even if it is significant, has only a minor effect on the heat transfer in a heat exchanger with clean plates, an unevenly distributed flow is, in many cases, unacceptable since the risk of fouling increases considerably. When fouling occurs, the heat transfer capacity of the heat exchanger is drastically reduced. Besides reducing the thermal efficiency, fouling may also have a detrimental effect on the quality of the product that has passed through the heat exchanger. Furthermore, more cleaning will be required and, in serious cases, unscheduled stoppages may be necessary.
One example of processes where the pressure drop across the heat transfer surface is small is climbing film evaporation.
To obtain a sufficient distribution across the flow area also in applications characterised by low pressure drops, the pattern of the flow area must be ‘open’, i.e. a sufficient flow should be obtained even without large pressure differences. For the purpose of distribution, the pattern should thus be ‘open’ in the transverse direction, and for the purpose of main flow, the pattern should be ‘open’ in the main flow direction. An ‘open’ pattern is obtained simply by making the plates as plane as possible and providing them with only a small number of local depressions. However, with only a small number of contact points, each contact point has to bear a considerable load and the portions of the plate located between the contact points are subjected to considerable bending loads.
One problem associated with prior art is the fact that there is no structure which in a completely satisfactory manner yields the desired distribution also at small pressure drops while providing a strong plate pack formed by the individual plates.
Known compromises between the two seemingly incompatible construction requirements present too many deficiencies in terms of either distribution or strength.